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IV/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION March,2017 ELECTRONICS AND INSTRUMENTATION ENGINEERING Eighth Semester OPTOELECTRONICS AND LASER INSTRUMENTATION (EI422) (Scheme of Evaluation) Time: Three Hours Maximum: 60 Marks Answer Question No.1 compulsorily. (1 x 12 = 12 M) Answer ONE question from each unit. 1. (a) Angle of incidence should be greater than Critical angle i.e., φ > φC. (b) NA= sin(θa). (c) Single mode fiber has a very small core diameter (2 - 10μm) and allows only one mode i.e., along the axis of the fiber. Multimode fiber has a greater core diameter ( ˃ 50μm ) and allows more number of modes. (d) () = { 1(1 − 2∆(/) ) 1 2 < () 1(1 − 2∆) 1/2 ≥ () (e) 632.8nm and 3.39μm. (f) Light amplification in the laser occurs when a photon colliding with an atom in the excited energy state causes the stimulated emission of a second photon and then both these photons release two more. Continuation of this process effectively creates avalanche multiplication, and when the electromagnetic waves associated with these photons are in phase, amplified coherent emission is obtained. This is called laser action. (g) Alloys of group III-V elements: Al, Ga, In, P, As, Sb etc. (h) In intrinsic optical fiber sensors the physical parameter acts on the fiber itself. In extrinsic optical fiber sensors there is change in the coupling characteristic between the feed fiber and return fiber. (i) Used in electrically hazardous environments, where conventional monitoring is difficult and expensive.
Transcript
Page 1: IV/IV B.Tech (Regular/Supplementary) DEGREE ...becbapatla.ac.in/html/scheme/mar2017/42/EI422.pdfIV/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION March,2017 ELECTRONICS AND INSTRUMENTATION

IV/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION

March,2017 ELECTRONICS AND INSTRUMENTATION ENGINEERING

Eighth Semester OPTOELECTRONICS AND LASER INSTRUMENTATION (EI422)

(Scheme of Evaluation)

Time: Three Hours Maximum: 60 Marks

Answer Question No.1 compulsorily. (1 x 12 = 12 M)

Answer ONE question from each unit.

1.

(a) Angle of incidence should be greater than Critical angle i.e., φ > φC.

(b) NA= sin(θa).

(c) Single mode fiber has a very small core diameter (2 - 10μm) and allows only one mode i.e., along the axis of the fiber. Multimode fiber has a greater core diameter ( ˃ 50μm ) and allows more number of modes.

(d) 𝑛(𝑟) = { 𝑛1(1 − 2∆(𝑟/𝑎)𝛼)1

2 𝑟 < 𝑎 (𝐶𝑜𝑟𝑒)

𝑛1(1 − 2∆)1/2 𝑟 ≥ 𝑎 (𝐶𝑙𝑎𝑑𝑑𝑖𝑛𝑔)

(e) 632.8nm and 3.39μm.

(f) Light amplification in the laser occurs when a photon colliding with an atom in the excited energy state causes the stimulated emission of a second photon and then both these photons release two more. Continuation of this process effectively creates avalanche multiplication, and when the electromagnetic waves associated with these photons are in phase, amplified coherent emission is obtained. This is called laser action.

(g) Alloys of group III-V elements: Al, Ga, In, P, As, Sb etc. (h) In intrinsic optical fiber sensors the physical parameter acts on the fiber itself. In extrinsic optical fiber sensors there is change in the coupling characteristic

between the feed fiber and return fiber. (i) Used in electrically hazardous environments, where conventional monitoring is

difficult and expensive.

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(j) Measurement of strain: Intrinsic fiber sensor (Pressure, vibration, sound): Stain acting on the fiber causes bending resulting in change in the intensity of light. Extrinsic fiber sensor (Pressure): The refractive index of a photo-elastic material changes with strain or pressure. (k) The penetration depth of light is more in an intrinsic semiconductor which results

greater absorption. (l) Change in the refractive index of the fiber when light is transmitted through a fiber.

UNIT-I 2.(a) Advantages of optical fibers: 3M

i. Enormous potential bandwidth. ii. Small size and weight.

iii. Electrical isolation. iv. Immunity to interference and crosstalk. v. Signal security.

vi. Low transmission loss. vii. Ruggedness and flexibility.

viii. System reliability and ease of maintenance. ix. Potential low cost.

Disadvantages of optical fibers: i. Installation Costs are High.

ii. Special Test Equipment is Often Required iii. Susceptibility to Physical iv. Fragility v. Opaqueness

vi. Requires Special Skills vii. Need Special Equipment

Block diagram of optical fiber communication system: 3M

Digital optical fiber link:

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(b) Given:n1=1.496,n2=1.40

Formula 2

2

2

10 )sin(. nnnNA a

For air n0=1 , (3M) NA=0.527, Acceptance angle, θa=sin-1(0.527)=31.80. For water refractive index at 200C is nw=1.33. (3M)

2

2

2

1)sin(. nnnNA aww

NA is same. Therefore, NA= 0.527

Acceptance angle ,θaw= )(sin

2

2

2

11

wn

nn =sin-1(0.527/1.33)=23.340

Or 3.(a)Given: Core diameter (2a) = 7.2μm.

Core refractive index,n1=1.46, Relative refractive index difference, Δ=1%=0.01.

Wavelength of light , λ=1.55μm.

(i) Normalized frequency,

V= 3 (3M)

(ii) β= 5.8922 X 106

Phase velocity ,

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18 sec1012/2/2/ radxcfp (3M)

(b) Attenuation: (3M)

The attenuation or transmission loss of optical fibers has proved to be one of the most important factors in bringing about their wide acceptance in telecommunications. As channel attenuation largely determined the maximum transmission distance prior to signal restoration, optical fiber communications became especially attractive when the transmission losses of fibers were reduced below those of the competing metallic conductors (less than 5 dB km−1).Signal attenuation within optical fibers, as with metallic conductors, is usually expressed in the logarithmic unit of the decibel. The decibel, which is used for comparing two power levels, may be defined for a particular optical wavelength as the ratio of the input (transmitted) optical power Pi into a fiber to the output (received) optical power Po from the fiber as:

Number of decibels (dB) = )/(log10 10 oi PP

This logarithmic unit has the advantage that the operations of multiplication and division reduce to addition and subtraction, while powers and roots reduce to multiplication and division. However, addition and subtraction require a conversion to numerical values which may be obtained using the relationship:

)10/(10 dB

o

i

P

P

In optical fiber communications the attenuation is usually expressed in decibels per unit length (i.e. dB km−1) following:

αdBL = )/(log10 10 oi PP

where αdB is the signal attenuation per unit length in decibels which is also referred to as the fiber loss parameter and L is the fiber length. Dispersion: (3M)

Dispersion of the transmitted optical signal causes distortion for both digital and analog transmission along optical fibers. When considering the major implementation of optical fiber transmission which involves some form of digital modulation, then dispersion mechanisms within the fiber cause broadening of the transmitted light pulses as they travel along the channel.

The pulse broadens and overlaps with its neighbors, eventually becoming indistinguishable at the receiver input. The effect is known as intersymbol interference (ISI). Thus an increasing number of errors may be encountered on the digital optical channel as the ISI becomes more pronounced. The error rate is also a function of the signal attenuation on the link and the subsequent signal-to-noise ratio (SNR) at the receiver. However, signal dispersion alone limits the maximum possible bandwidth attainable with a particular optical fiber to the point where individual symbols can no longer be distinguished.

For no overlapping of light pulses down on an optical fiber link the digital bit rate BT must be less than the reciprocal of the broadened (through dispersion) pulse duration (2τ). Hence:

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UNIT-II

4.(a) Spontaneous Emission and Stimulated Emission: (6M)

The interaction of light with matter takes place in discrete packets of energy or quanta,called photons. Furthermore, the quantum theory suggests that atoms exist only in certain discrete energy states such that absorption and emission of light causes them to make a transition from one discrete energy state to another. The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the higher energy state E2 and the lower energy state E1 by the expression:

where h = 6.626 × 10−34 J s is Planck’s constant. These discrete energy states for the atom may be considered to correspond to electrons occurring in particular energy levels relative to the nucleus. Hence, different energy states for the atom correspond to different electron configurations, and a single electron transition between two energy levels within the atom will provide a change in energy suitable for the absorption or emission of a photon.

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The emission process can occur in two ways: (a) by spontaneous emission in which the atom returns to the lower energy state in an entirely random manner; (b) by stimulated emission when a photon having an energy equal to the energy difference between the two states (E2 − E1) interacts with the atom in the upper energy state causing it to return to the lower state with the creation of a second photon.

Spontaneous emission:

The increased concentration of minority carriers in the opposite type region in the forward- biased p–n diode leads to the recombination of carriers across the bandgap. The normally empty electron states in the conduction band of the p-type material and the normally empty hole states in the valence band of the n-type material are populated by injected carriers which recombine across the bandgap. The energy released by this electron–hole recombination is approximately equal to the bandgap energy Eg.

Excess carrier population is therefore decreased by recombination which may be radiative

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or nonradiative.In nonradiative recombination the energy released is dissipated in the form of lattice vibrations and thus heat. However, in band-to-band radiative recombination the energy is released with the creation of a photon with a frequency where the energy is approximately equal to the bandgap energy Eg and therefore:

This spontaneous emission of light from within the diode structure is known as electroluminescence. The light is emitted at the site of carrier recombination which is primarily close to the junction, although recombination may take place through the hole diode structure as carriers diffuse away from the junction region.

Stimulated emission:

Carrier population inversion is achieved in an intrinsic (undoped) semiconductor by the injection of electrons into the conduction band of the material.Electrons injected into the material fill the lower energy states in the conduction band up to the injection energy or the quasi-Fermi level for electrons. Since charge neutrality is conserved within the material, an equal density of holes is created in the top of the valence band by the absence of electrons. Incident photons with energy Eg but less than the separation energy of the quasi-Fermi levels Eq = EFc − EFv cannot be absorbed because the necessary conduction band states are occupied. However, these photons can induce a downward transition of an electron from the filled conduction band states into the empty valence band states, thus stimulating the emission of another photon. The basic condition for stimulated emission is therefore dependent on the quasi-Fermi level separation energy as well as the bandgap energy and may be defined as:

Population inversion may be obtained at a p–n junction by heavy doping (degenerative doping) of both the p- and n-type material. Heavy p-type doping with acceptor impurities

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causes a lowering of the Fermi level or boundary between the filled and empty states into

the valence band. Similarly, degenerative n-type doping causes the Fermi level to enter the

conduction band of the material.

At high injection carrier density in such a junction there exists an active region near the depletion layer that contains simultaneously degenerate populations of electrons and holes. For this region the condition for stimulated emission is satisfied for electromagnetic radiation of frequency Eg/h < f < (EFc − EFv)/h. Therefore, any radiation of this frequency which is confined to the active region will be amplified.

(b) CO2 Laser:

In CO2 vibrational transitions take place. Lasing emits two wavelengths 10.6μm and 9.6μm.

Addition of N2 gas to CO2 gas increases the efficiency.In CO2 energy levels are very close to

the ground level and hence a large portion of the input energy is converted into the o/p

laser energy, resulting in very efficiencies. The output power is several watts to several

hundred watts. Its applications in industries are welding, cutting, hole drilling etc.

The carbon dioxide molecule consisting of a central carbon atom with two oxygen atoms attached one on either side. Such a molecule can vibrate in the three independent modes of vibration. These correspond to the symmetric stretch, the bending, and the asymmetric stretch modes. Each of these modes is characterized by a definite frequency of vibration. According to basic quantum mechanics these vibrational degrees of freedom are quantized, i.e., when a molecule vibrates in any of the modes it can have only a discrete set of energies.

(3M)

(3M)

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The laser transition at 10.6 μm occurs between the (001) and (100) levels of carbon dioxide. The excitation of the carbon dioxide molecules to the long-lived level (001) occurs both through collisional transfer from nearly resonant excited nitrogen molecules and also from the cascading down of carbon dioxide molecules from higher energy levels.

Or

5.(a)Holography:

An ordinary photograph represents a two-dimensional recording of a three dimensional scene. The emulsion on the photographic plate is sensitive only to the intensity variations, and hence while a photograph is recorded, the phase distribution which prevailed at the plane of the photograph is lost. Since only the intensity pattern has been recorded, the three-dimensional character (e.g., parallax) of the object scene is lost.

During the recording process, one superimposes on the wave (emanating from the object) another coherent wave called the reference wave .The two waves interfere in the plane of the recording medium and produce interference fringes. This is known as the recording process. The interference fringes are characteristic of the object and the recording medium records the intensity distribution in the interference pattern.This interference pattern has recorded in it not only the amplitude distribution but also the plane of the object wave. Thus, let

represent the field produced due to the object wave at the plane of the recording medium; O0 (x, y) is the amplitude part and φ (x, y) the phase part. Similarly, let

represent the field produced due to the reference wave at the recording medium.Usually the reference wave is an obliquely incident plane wave, in which case A is a constant. The total field produced at the recording medium is

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and the intensity pattern recorded by the recording medium would be where we have omitted a constant of proportionality and have carried out a time averaging. It can immediately be seen from the above that the recorded intensity distribution has the phase of the object wave φ (x, y) embedded in it. This hologram has little resemblance to the object. It has in it a coded form of the object wave. The technique by which one reproduces the image is termed reconstruction.In the reconstruction process, the hologram is illuminated by a wave called the reconstruction wave; this reconstruction wave in most cases is similar to the reference wave used for recording the hologram.When the hologram is illuminated by the reconstruction wave, various wave components emerge from the hologram, one of which is the object wave itself. In order to show this, we see that when the exposed recording medium is developed, then one, in general, gets a transparency, with a certain transmittance. Under proper conditions, the amplitude transmittance of the hologram can be made to be linearly proportional to I (x, y). Thus apart from some constants, one can write for the amplitude transmittance of the hologram. (3M) Recording

Reconstruction

(3M)

(b)Biomedical applications of laser: (6M)

Explanation of Eye Surgery, Treatment of cancer, Dentistry, Dermatology etc

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UNIT-III

6.(a)Mach-Zehnder interferometer:

Based on the principle of interference of coherent monochromatic light propagating

in a strained fiber.Consists of reference fiber and strained fiber. Pressure causes variation in

length, refractive index, core diameter of the fiber.This provides a phase difference between

light emitted from the two fibers which generates interference patterns. (3M)

(3M)

b)

(6M)

SPONTANEOUS EMISSION STIMULATED EMISSION

In-Coherent Light Coherent Light

Wide Line Width Narrow Line Width

Dispersed In Space Highly Directional

Low Intensity High Intensity

Device: Led Device: Laser

Current Density Required is Small Large Current Density is Required

LASER REFERENCE FIBER

MEASUREMENT

ENVIRONMENT

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Or

7.

(a) Faraday Current monitor:

(3M)

Principle: Modulation of Polarization of light by a longitudinal magnetic field

produced by current through a conductor.This phenomenon is called as magneto-optic

effect.The polarization is split into two orthogonal components using Wollatson prism.These

orthogonal components generate current I1 and I2.The ratio( I1-I2)/(I1+I2) is proportional to

the current passing through the electrical conductor. (3M)

b)Double Hetero junction LED:

The device consists of a p-type GaAs layer sandwiched between a p-type AlGaAs and an n-type AlGaAs layer. When a forward bias is applied electrons from the n-type layer are injected through the p–n junction into the p-type GaAs layer where they become minority carriers. These minority carriers diffuse away from the junction ,recombining with majority carriers (holes) as they do so. Photons are therefore produced with energy corresponding to the bandgap energy of the p-type GaAs layer. The injected electrons are inhibited from diffusing into the p-type AlGaAs layer because of the potential barrier presented by the p–p heterojunction (see Figure 7.2(b)). Hence, electroluminescence only occurs in the GaAs junction layer, providing both good internal quantum efficiency and high-radiance emission. Furthermore, light is emitted from the device without reabsorption because the bandgap energy in the AlGaAs layer is large in comparison with that in GaAs. The DH structure is therefore used to provide the most efficient incoherent sources for application within optical fiber communications. Nevertheless, these devices generally exhibit the previously

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discussed constraints in relation to coupling efficiency to optical fibers. This and other LED structures are considered in greater detail in the following section. (3M)

(3M)

UNIT-IV

8.(a)Acousto-optic modulator:

θ

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An acousto-optic modulator (AOM), also called a Bragg cell, uses the acousto-optic

effect to diffract and shift the frequency of light using sound waves (usually at radio-

frequency). A diffracted beam emerges at an angle θ that depends on the wavelength of the

light λ relative to the wavelength of the sound Λ . (3M)

msin

Electro-optic modulator:

Electro-optic modulator (EOM) is an optical device in which a signal-controlled element exhibiting the electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the beam. Modulation bandwidths extending into the gigahertz range are possible with the use of laser-controlled modulators.The electro-optic effect is the change in the refractive index of a material resulting from the application of a DC or low-frequency electric field. (3M)

b) Avalanche photodiode: The second major type of optical communications detector is the avalanche

photodiode (APD). This has a more sophisticated structure than the p–i–n photodiode in order to create an extremely high electric field region (approximately 3 × 105 V cm−1). Therefore, as well as the depletion region where most of the photons are absorbed and the primary carrier pairs generated, there is a high-field region in which holes and electrons can acquire sufficient energy to excite new electron–hole pairs. This process is known as impact ionization and is the phenomenon that leads to avalanche breakdown in ordinary reverse-biased diodes. It often requires high reverse bias voltages (50 to 400 V) in order that the new carriers created by impact ionization can themselves produce additional carriers by the same mechanism. More recently, however, it should be noted that devices which will operate at much lower bias voltages (15 to 25 V) have become available. (3M)

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Response time is limited by three factors:

(i) Transit itme across the absorption region. (ii) Time for avalanche multiplication. (iii) RC time constant of junction capacitance of diode and load. (3M)

Or

9.(a) p-i-n photodiode: Used for operation at long wavelengths. Light penetrates deeply into the semiconductor. Wide depletion region. Lightly doped n-type acts like intrinsic layer. (3M) Avalanche photodiode: High voltages are applied.(50-400V reverse biasing voltage) Avalanche break down takes place. Consists of high electric field region.(3 x 105Vcm-1) (3M) (b) pin photodiode: In order to allow operation at longer wavelengths where the light penetrates more deeply into the semiconductor material, a wider depletion region is necessary. To achieve this the n-type material is doped so lightly that it can be considered intrinsic, and to make a low resistance contact a highly doped n-type (n+) layer is added. This creates a p–i–n (or PIN) structure where all the absorption takes place in the depletion region. (3M)

(3M)

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SCHEME OF EVALUATION

SUBJECT: OPTOELECTRONICS AND LASER INSTRUMENTATION

CODE: EI-422

ACADEMIC YEAR :2016-17

Prepared by M.V.N.CHAKRAVARTHI Head of the Department Assistant Professor, Department of EIE, BEC.


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